1. Field of the Invention
The present invention relates to a photosensitive resin composition for producing a relief printing plate. More particularly, the present invention is concerned with a photosensitive resin composition containing a specific hindered amine having a piperidine ring, which composition is capable of providing a printing plate having excellent printing resistance. The photosensitive resin composition is advantageously used as a material for preparing a printing plate, particularly a printing plate for use in flexographic printing, printing of a corrugated board, stamping, or the like.
2. Discussion of Related Art
In recent years, rapid progress has been made with respect to photosensitive resin compositions. These photosensitive resin compositions are now widely used for producing photoreliefs. For example, an image relief to be used for preparing a relief printing plate and the like, or an image resist to be used in etching for preparing a lithographic plate, a gravure, a printed circuit, an integrated circuit, a shadow mask and the like. Also, the photosensitive resin compositions are now widely used as a photosensitive vehicle for a coating composition, an adhesive or a printing ink.
In the preparation of a printing plate by the use of a photosensitive resin composition, the photosensitive resin compositions are generally photopolymerized by irradiation of actinic rays, such as ultraviolet rays, to form a three dimensional structure so that the photosensitive resin compositions provide cured areas which are insoluble in water, an organic solvents and the like. When a photosensitive resin composition is cured to form a printing plate having a relatively large thickness, such as a relief printing plate, the composition must be exposed to actinic rays for a prolonged period of time until even the inner portion of the composition is sufficiently cured. Consequently, the surface portion of the photosensitive resin composition is inevitably irradiated by actinic rays of a ray quantity which would exceed the amount sufficient for curing only the surface portion of the composition. Thus, the surface portion of the composition is excessively irradiated by the actinic rays. As a result, the surface portion cured by photopolymerization undergoes photodeterioration due to the excessive irradiation of by the actinic rays so that the tensile elongation at break and the tensile strength at break of the resultant printing plate are likely to be lowered, thereby leading to the printing plate having poor printing resistance. In other words, the photocured surface portion of the resin composition becomes brittle. That is, the mechanical strengths, such as toughness defined as the product of tensile elongation at break multiplied by tensile strength at break, of the resultant printing plate are lowered so that the relief of the printing plate is likely to be broken during practical printing operation.
On the other hand, when the quantity of actinic rays for irradiation is reduced, the inner portion of the photosensitive resin composition is not sufficiently cured so that the resultant printing plate is insufficient in mechanical properties, such as tensile elongation at break and tensile strength at break, thereby causing the ultimate printing plate to have poor printing resistance.
In this connection, it is noted that the photocured resin layer of a relief printing plate generally has a thickness as large as from 0.5 to 10.0 mm. Particularly, in the printing of a corrugated board, since the corrugated board is very soft, the printing plate to be used generally has a thickness as large as from 3 to 10 mm and the relief layer thereof has a thickness of from 1 to 9 mm which is large as compared to printing plates which are to be used in printings other than relief printing. Due to such a large thickness of the desired relief layer, non-uniform crosslinking of the photosensitive resin along the thickness of the final relief layer brings about a large problem in preparing a relief printing plate from a photosensitive resin composition.
As apparent from the above, it is difficult to prepare a relief printing plate having a good balance of mechanical strength as between the surface portion and the inner portion.
There have been proposals for overcoming the non-uniformity in curing degree between the surface portion and the inner portion of a photosensitive resin composition which is present due to the difference in light penetration within the photosensitive resin. For example, in the field of ultraviolet absorbing coatings having a thickness of 0.1 mm or less, it was proposed to cure the surface portion of photosensitive resin composition with ultraviolet rays having a relatively short wavelength and cure the inner portion of the composition with ultraviolet rays having a relatively long wavelength (see U.S. Pat. No. 4,182,665). This technique of using actinic rays having different wavelengths has applied in the preparation of a relief printing plate, since the depth which actinic rays can reach in a photosensitive resin composition is very small. Thus, when the thickness of a layer of a photosensitive resin composition exceeds 1 mm, such a technique cannot provide a good balance of mechanical strengths as between the surface portion and the inner portion of the ultimate relief printing plate. Therefore, the above-mentioned problem cannot be overcome by this technique.
It has also been proposed to incorporate an ultraviolet absorber or a conventional radical quencher into a photosensitive resin composition or to coat the surface of a photosensitive resin composition with an ultraviolet absorber or a conventional radical quencher. However, these proposals have not been successful in providing a good balance of mechanical strengths between the surface portion and the inner portion of a relief printing plate. Therefore, the above-mentioned problem also cannot be overcome by these proposals.
Meanwhile, an ultraviolet absorber and a cyclic hindered amine have been known as a light stabilizer for preventing the photodeterioration of a polymer. For improving the sunlight resistance and preventing the discoloration of a polymer, it has been attempted to incorporate an ultraviolet absorber or a cyclic hindered amine into a lacquer, a coating composition, a molding material, a coating material or an ink (see U.S. Pat. Nos. 4,279,720 and 4,284,485 and Japanese Patent Application Laid-Open Specification Nos. 59-168073 and 59-184227). However, these attempts were made in order to improve the sunlight resistance and to prevent the discoloration of a polymer by sunlight, in particular, in order to improve the sunlight resistance and to prevent discoloration of a polymer for a very long period of time, e.g., several years.
Up to the present time, no attempt has been made to prevent a photosensitive resin from being photodeteriorated or to prevent the mechanical strengths from being lowered by modifying the photosensitive resin composition during the irradiation of strong actinic rays having a main wavelength zone in the ultraviolet region for a short period of time, i.e., at most several tens of minutes, in order to prepare a relief printing plate.